feat: add tests and CI coverage for agentcore-sdlc-inspector

## Description

The `agentcore-sdlc-inspector/` POC currently has no tests and is not included in CI. As the inspector graduates from POC to a maintained tool, it needs test coverage and CI integration.

## Scope

### API Tests (`agentcore-sdlc-inspector/api/`)

- [ ] Unit tests for event stream parser (`aws/agentcore.ts`) — mock binary event stream frames, verify stdout/stderr/exitCode extraction
- [ ] Unit tests for DDB scan logic (`aws/dynamodb.ts`) — mock DynamoDB responses, verify grouping/filtering
- [ ] Unit tests for SigV4 signing helper (`aws/sign.ts`) — verify request construction and header inclusion
- [ ] Integration tests for routes:
- `GET /api/health` → returns `{ ok: true, checked_at: string }`
- `GET /api/sessions?window=0` → verifies falsy-zero fix (window=0 triggers full scan)
- `POST /api/check-claude` → mock AgentCore response, verify CLAUDE_RUNNING/NOT_RUNNING parsing
- `POST /api/stop-session` → mock AgentCore response, verify stopped/not-stopped return
- `POST /api/runtime-lifecycle` → mock uptime response, verify seconds parsing
- [ ] Test framework: vitest or jest (pick one, pin version)

### Frontend Tests (`agentcore-sdlc-inspector/web/`)

- [ ] Component tests for `SessionCard` — render with mock data, verify buttons/badges/status
- [ ] Component tests for `SessionList` — verify grouping logic (skipped invocations in history, non-skipped as primary)
- [ ] Component tests for `App` — verify time window filter, auto-refresh, dark mode toggle
- [ ] Test framework: vitest + @testing-library/react

### CI Integration

- [ ] Add to `.github/workflows/ci.yml` (or new workflow):
```yaml
inspector:
runs-on: ubuntu-latest
defaults:
run:
working-directory: agentcore-sdlc-inspector
steps:
- uses: actions/checkout@v4
- uses: actions/setup-node@v4
with:
node-version: '20'
- run: npm install
- run: cd api && npx tsc --noEmit
- run: cd web && npx tsc --noEmit
- run: cd api && npm test
- run: cd web && npm test
```
- [ ] Add `test` script to both `api/package.json` and `web/package.json`
- [ ] Ensure `npx cdk synth --quiet` still passes (inspector not included in CDK build)

## Files to Create / Modify

```
agentcore-sdlc-inspector/api/src/__tests__/ NEW — API unit/integration tests
agentcore-sdlc-inspector/web/src/__tests__/ NEW — Component tests
agentcore-sdlc-inspector/api/package.json MODIFY — add test deps + script
agentcore-sdlc-inspector/web/package.json MODIFY — add test deps + script
.github/workflows/ci.yml MODIFY — add inspector job
```

## Test Priorities (highest first)

1. Event stream parser — most complex logic, most likely to regress
2. Falsy-zero window handling — already caught one bug here
3. Session grouping (skipped vs. non-skipped primary selection)
4. Route handlers (mock AWS SDK, verify request/response shapes)
5. Frontend component rendering

## Constraints

- Pin all test dependencies to exact versions
- Mock AWS SDK calls (never hit real DynamoDB/AgentCore in tests)
- Tests must pass without AWS credentials configured
- TypeScript strict mode for test files
- No snapshot tests (they create noise on Cloudscape version bumps)

Start with agentcore-sdlc-inspector/api/src and web/src, then review api/package.json, web/package.json, and .github/workflows/ci.yml to choose and pin the test frameworks. Add the listed API and component tests, scripts, and inspector CI job; done means both test suites and type checks pass without AWS credentials and cdk synth still succeeds.

Written by the indexing model from the issue text.